German published patent application DE-OS No. 24 42 373 discloses an electrically controlled intermittent fuel-injection apparatus wherein injection signals are generated on the basis of rotational speed and throttle flap position. In this application, the throttle flap position is sensed by means of a potentiometer.
With a view to accomplishing optimum metering of fuel, it is necessary for the throttle flap position to be detected with a high degree of accuracy, particularly in the range of relatively small opening angles of the throttle flap. For example, for adjusting idle speed, an idle position of the throttle flap has to be detected for which purpose an idle switch is conventionally used. However, the idle stop is subject to changes due to both idle-speed adjustment and mechanical wear of the limit stop. It is, however, necessary that the idle position of the throttle flap be detected within a very small angular range of about 0.3.degree. because it is only in this range that the air flows are sufficiently small to keep the torque change within tolerable limits when the fuel supply is cut off or resumed by means of the overrun mode of operation.
German published patent application DE-OS No. 34 28 879 discloses a digital method for detecting the idle condition; in this known method, the slider path of the position sensor, which is configured as a potentiometer, is subdivided into increments. A stored limit value is followed-on with a specific time constant in dependence on measured values. For follow-on, however, a constant angular swing between minimum value and maximum value has to be taken into consideration, with the entire measuring range eventually being followed on. This method is not particularly suitable, especially for non-linear potentiometers.